Uplink multi-user MIMO (UL-MU-MIMO) transmission under study in IEEE802.11ax requires an access point (AP) to transmit a trigger frame for uplink transmission to a plurality of wireless terminals for which uplink transmission is permitted. A wireless terminal which has received the trigger frame performs uplink transmission in a predetermined time after receiving the trigger frame. Thus, uplink transmission is performed simultaneously from the plurality of wireless terminals.
Even when the AP permits uplink transmission for a wireless terminal which does not store uplink transmission data, data is not transmitted from the wireless terminal, and therefore when uplink transmission is permitted for such a wireless terminal, the corresponding amount of wireless resources becomes wasted. For this reason, a method is conceived for the AP to inquire about whether or not there is uplink transmission data in advance before UL-MU-MIMO starts. The AP inquires about whether or not there is uplink transmission data in advance before UL-MU-MIMO starts, and can thereby reliably give permission to the wireless terminal which stores the uplink transmission data.
On the other hand, such an inquiry may delay the start of UL-MU-MIMO and lead to an increase of overhead (decrease in MAC efficiency).
Furthermore, at the time of UL-MU-MIMO transmission, as the number of selectable wireless terminals from which the AP can select candidates of wireless terminals for which uplink transmission is permitted increases, the number of combined candidates increases. For this reason, it is possible to select combination wireless terminals more suitable for UL-MU-MIMO transmission (from the standpoint of user correlation, frame length, power difference or the like), but this requires inquiries to make more appropriate combinations for many wireless terminals, leading to an increase of overhead accordingly.